We just covered:

I  Introduction
II  The Structure of Amino Acids & Protein
III    FUNCTIONS of Protein
IV  Protein in the Diet- Quality
V  Problems with Protein Quantity

Now we'll cover:

VI     Digestion of Protein
VII    Using Amino Acids


VI  Digestion of Protein

The first part of chemical digestion of proteins begins in the stomach, when hydrochloric acid denatures proteins.  Denaturation is when the protein uncoils and loses its natural shape, like illustrated below.  Keep in mind the protein will no longer function once it is denatured, remember shape determines function.

Acid is one agent that can denature proteins, but other agents, like heat, can also denature proteins.  This is what happens when you cook an egg.  The heat uncoils the proteins in the egg.  This animation, "Heat Changes Protein Structure: Frying an Egg," demonstrates this.  Click on the narrated box (as shown below) to get started with the animation.

Once proteins are denatured or uncoiled, then enzymes have an easier time facilitating the breakdown of proteins through enzymatic digestion.  Enzymatic digestion breaks the protein into smaller peptide chains and ultimately down into single amino acids, which are absorbed into the blood

The proteases and peptidases listed above are enzymes.  Enzymes for protein digestion are made in the stomach, pancreas, and intestinal cells.  Enzymatic digestion begins in the stomach and ends in intestinal cells

Below is an overview of protein digestion:



Now we have discussed 3 macronutrients (carbohydrates, triglycerides, and protein).  We have discussed where you find these nutrients in foods, if they need to be enzymatically digested, and what is absorbed after enzymatic digestion.  Let's practice an application question that you will see through out the term within study questions and exams.

Consider a breakfast of oatmeal, 2% milk, butter and sugar.  What needs to be enzymatically digested, and what is absorbed into intestinal cells after digestion?

oatmeal starch




glycerol, fatty acids, and monoglycerides

amino acids

milk lactose



glucose, galactose

glycerol, fatty acids, and monoglycerides

amino acids

sugar sucrose glucose, fructose
butter triglycerides glycerol, fatty acids, and monoglycerides

Notice that both whole foods (oatmeal and milk) have all 3 macronutrients.  This will always be the case.  The two refined foods do not.  Concentrated sugar is just carbohydrate, and concentrated fat is just triglycerides.

Oatmeal would also have fiber, but it is not digested or absorbed.

VII    Using Amino Acids

Once the digestive system has enzymatically digested a protein in food so that it has been broken down to amino acids,  the amino acids are absorbed into the villi (then the blood), as discussed above.  

The amino acids then travel in the blood to cells all over the body.

The next section describes what happens when an amino acid from a protein in food gets to a cell.

A.    If there's NOT enough glucose available to the cell, the amino acid will be stripped of its nitrogen, which is excreted in urine.  The image below is again the amino acid, aspartic acid.

Aspartic Acid

It is the liver that strips the nitrogen off and the kidney which puts it in urine for excretion.  (That's why a baby's diaper smells like ammonia, which is NH3.)

Once the NH2 has been stripped off, the rest of it will be 
rearranged into glucose for use by brain, if needed.  


If actual glucose isn't needed, it will be burned as fuel (it does not have to be turned into glucose to be burned as fuel).

B.    If there IS enough glucose available to the cell, the amino will be used to
  • make a protein the cell needs, like tissue proteins, muscles or enzymes or it will be used   
  • make another amino acid the cell needs     
  • make other small nitrogen-containing compounds such as the vitamin, niacin
If none of those are needed, it would be rearranged and stored as fat (this would also mean calorie intake exceeds calorie expenditure).

C.    If Carla has been following a high protein and low carbohydrate diet,
what would be happening is part A.  (above).  In other words, the amino groups in the amino acids of much of the protein she's eating would be split off and the remainder of the amino acid would be changed to glucose for the brain or burned as a fuel source.  However, protein is still being used for part B. as well, but the majority of it would be burned for energy.  The extra protein would only be stored as fat, if total Calories were also in excess.  Typically with a high protein diet, there is less variety, so people tend to eat less Calories than they need, which means protein would not be stored as fat.

D.    If Don has been eating a diet with plenty of foods with carbohydrate & protein,
what would be happening is part B.  (above)  In other words, the protein would be primarily used to supply amino acids to build body proteins.    If he is consuming more calories than he needs, some protein would be changed to fat.  With a diet that has plenty of carbohydrate, very little protein would be burned for energy part A.   

E.    The body can use either liver glycogen or muscle protein to make glucose for the brain.  The body cannot change significant amounts of body fat (specifically fatty acids) into glucose for the brain, and muscles hoard their glycogen, they don't share with the brain.


Now you're ready for 6B